First the 5.7 was made to be a PDW and pistol round to replace the 9mm and able to defeat body armor when the real 5.7 ammo is used(not available to public). The guys making ARs thought a rifle/pistol combination using the same ammo would be a good idea. The ammo is not as effective as 5.56 and more expensive.
It is lighter weight and the mags have a much higher capacity.

So if going that route to replace a 9mm sub gun yes, to replace the 5.56? no IMO.

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